How to prolong the life of an external hard drive?

If you put an external harddrive next to the server/desktop and:

• Do not carry it around in your bag like it is a piece of rock.
• Do not toss it around like a piece of fruit.
• Do not expose it to cold wet outside weather followed by dry hot (room) temperatures
• Do not unplug it and pick it up while it still is spinning.
• And mount it in a proper case with a decent PSU and sufficient cooling,
• ...

then I see no reason why they should not last just as long as internal drives.

Ofc, there is a reason why people buy external drives. And they often do get exposed to one or more of the conditions I mentioned above.

The primary reason I've come across for external hard drives dieing earlier is due to insufficient cooling in the enclosure; heat will kill a drive if allowed to.

Many of the externals I've seen are too cheap (and too light) to be just a regular HDD in an enclosure, so if you want a long-lived hdd, buy the same hdd you would put in a desktop, and an enclosure for it. Keep it in one place, as far away from humans as possible, and it will likely enjoy a full lifespan. Check your SMART stats a couple times a year to ensure its still healthy.